Incidental bycatch mortality and fishing restrictions: impacts on juvenile survival in the Endangered Saimaa ringed seal Pusa hispida saimensis

Abstract

Incidental bycatch, mostly in gillnets used for recreational fishing, is a critical mortality factor for the Endangered Saimaa ringed seal Pusa hispida saimensis. Bycatch particularly affects juveniles, and therefore fishing has been restricted by banning gillnets during the most critical juvenile dispersal period in spring. In addition, the most harmful gear types for ringed seals of all ages are forbidden year-round. The springtime restriction areas have increased significantly since the 1990s, now covering over 90% of the pups’ birth sites. However, the impacts of fishing restrictions on incidental bycatch mortality have not previously been evaluated statistically. The present estimates of incidental bycatch levels in relation to fishing restrictions since the 1990s suggest that the springtime gillnet fishing ban resulted in an increase in population size by 20% between 1991 and 2013 (60 individuals given an average estimated stock size of 355 in 2013). In addition, the estimated critical period for juvenile survival in relation to fishing operations appeared to be the first 15 mo, which is a much longer time than previously expected, suggesting that stationary gillnet fishing during late winter months may cause a secondary peak in seal mortality. One unwanted estimated side effect of the seasonal ban was a slightly increased incidental estimated bycatch level immediately after the end of the springtime fishing restriction period. The estimated bycatch peaked in 2000-2005, with an average of 13.3 juveniles annually and decreased thereafter, mainly in 2009-2013, with an average of 6.3 juveniles annually.